Like other American cities in the late 19th century, Baltimore had grown so quickly its supply system was unable to provide city residents with a dependable supply of water. Two reservoirs built outside the city helped increase capacity, but heavy rainfalls in the largely agricultural area tended to foul this additional water supply. City officials elected to construct a holding reservoir within the city - contained by an earthen dam - where silty water would be allowed to settle. No such project had ever been undertaken in the United States.
"The roundhouse is an amazing survivor of an important era in American engineering and architectural history. Eric DeLony, chief of the National Park Service's Historic American Engineering Record, has called it 'the most important surviving cast-iron framed building in North America.'"
From: The Baltimore and Ohio Railroad Martinsburg Shop Complex: Historic Structure Report. By John P. Hankey, August, 2000.